1. Field of the Invention
The present invention relates to a technology for driving an OLED (organic light emitting diode) display panel, and more particularly, to a circuit and a method for driving an OLED display, in which a smaller number of sample/hold circuits are used when detecting a change in a threshold voltage of an OLED and performing an automatic compensation process.
2. Description of the Related Art
FIG. 1 is a block diagram schematically illustrating an OLED display in accordance with the related art. Referring to FIG. 1, the OLED display includes a display panel 10, a gate driver 20, a source driver 30, and a threshold voltage detection control unit 40. The display panel 10 displays an image by using pixels 11 arranged at intersection areas of gate lines GL and data lines DL. The gate driver 20 drives the gate lines GL1 to GLn of the display panel 10. The source driver 30 drives the data lines DL1 to DLn of the display panel 10. The threshold voltage detection control unit 40 precharges OLEDs of the display panel 10 and then detects threshold voltages thereof.
In the display panel 10, the pixels 11 including the OLEDs are arranged in a matrix form, and the OLEDs emit light corresponding to data signals supplied from the data lines DL1 to DLn when gate signals are supplied to the gate lines GL1 to GLn.
To this end, the gate driver 20 sequentially supplies the gate signals to the gate lines GL1 to GLn on the display panel 10 to sequentially drive the gate lines GL1 to GLn. The source driver 30 converts digital data signals supplied from an outside into analog data signals, and supplies the analog data signals to the data lines DL1 to DLn in synchronization with the gate signals.
The driving operation of the pixels 11 arranged in the matrix form on the display panel 10 will be described below in more detail.
The gate driver 20 sequentially outputs the gate signals to the gate lines GL1 to GLn of the display panel 10. In synchronization with this, the source driver 30 outputs the data signals to the data lines DL1 to DLn.
Switching transistors TFT-S on the first horizontal line are turned on by the gate signal supplied to the first gate line GL1. Thus, the data signals supplied through the data lines DL1 to DLn are supplied to gates of driving transistors TFT-D through the switching transistors TFT-S, so that the driving transistors TFT-D are turned on. Consequently, a driving current corresponding to the data signals is supplied to the OLEDs through the driving transistors TFT-D, so that the OLEDs emit light with a predetermined brightness. However, the data signals supplied through the switching transistors TFT-S are charged in a capacitor C, which is connected between the gate and source of the driving transistor TFT-D, for one frame time. Therefore, since the driving transistor TFT-D is turned on for one frame, a corresponding OLED continuously emits light for one frame.
Then, since OLEDs on the other horizontal lines sequentially emit light through the above-described process, all OLEDs of the display panel 10 emit light for one frame. Such an operation is continuously performed over a predetermined frame per second.
The OLED produces one of unique colors RGB and produces a target color through a combination with other OLEDs in a unit pixel.
However, the OLEDs gradually deteriorate with the passage of time, resulting in a change in threshold voltages Vth thereof. Therefore, even if the same driving current is supplied to the OLEDs, the brightness of the OLEDs is gradually changed as they are used for a long time.
In this regard, data signals are subject to a compensation process in correspondence with the change in the threshold voltages Vth of the OLEDs, so that the OLEDs always emit light with a constant brightness. Such a conventional threshold voltage compensation operation will be described below with reference to FIG. 2.
In the display panel 10, anodes of OLEDs on each horizontal line are connected to corresponding data lines through transistors TFT-V for threshold voltage detection, respectively. Gates of the transistors TFT-V for threshold voltage detection are commonly connected to threshold voltage compensation control lines CL of the threshold voltage detection control unit 40.
The source driver 30 includes sample/hold circuits S/H1 to S/Hn, which sample/hold the threshold voltages Vth of the OLEDs detected through the transistors TFT-V for threshold voltage detection, provided corresponding to the number of the data lines DL1 to DLn, and an A/D (analog-to-digital) converter 31 for converting the analog threshold voltages sampled/held through the sample/hold circuits S/H1 to S/Hn into digital signals and storing the digital signals in a memory.
Before the OLED display is powered on and an image is displayed on the display panel 10 or in a standby state, the threshold voltage detection control unit 40 sequentially outputs a control signal to the threshold voltage compensation control lines CL1 to CLn installed corresponding to the gate lines (or the horizontal lines) GL1 to GLn, so that the transistors TFT-V for threshold voltage detection on a corresponding horizontal line are sequentially turned on.
When the control signal is supplied to the first threshold voltage compensation control line CL1 and the transistors TFT-V for threshold voltage detection on the first horizontal line are turned on, the source driver 30 outputs precharge voltages to the data lines DL1 to DLn through buffers BUF1 to BUFn. Thus, the precharge voltages are supplied to the anodes of corresponding OLEDs through the transistors TFT-V for threshold voltage detection.
After a predetermined time lapses and the precharge voltages are sufficiently discharged from the OLEDs, the sample/hold circuits S/H1 to S/Hn sample/hold the threshold voltages Vth of the OLEDs which are detected through the transistors TFT-V for threshold voltage detection and a corresponding data line DL. The analog threshold voltages Vth sampled/held in this way are converted into digital signals through the A/D converter 31 and stored in a memory.
Then, the above-described threshold voltage detection operation is sequentially performed with respect to a subsequent horizontal line, and the threshold voltages Vth of corresponding OLEDs are converted into digital signals and stored in the memory through the above-described process.
After the above-described threshold voltage detection operation is completed, in an image display mode of the display panel 10, when data signals corresponding to RGB data supplied from an outside are outputted from the source driver 30 to the OLEDs through the data lines DL1 to DLn, the data signals are outputted after being compensated by variation in the original threshold voltage level with reference to the threshold voltages stored in the memory.
Consequently, the OLEDs always emit light with a constant brightness regardless of a change in the threshold voltages.
However, in the threshold voltage compensation circuit of the conventional OLED display, the sample/hold circuits corresponding to the number of the data lines are used when detecting threshold voltages in order to compensate for the threshold voltages of the OLEDs, resulting in an increase in a chip size and current consumption.